Methionine Transfer Ribonucleic Acids of Avian Myeloblastosis Virus

Abstract

Chick-embryo cells contain four isoaccepting species of methionine transfer RNA (I-IV). One species (I) is the initiator, tRNA'et, and the others (II, III, and IV) are the donors of internal methionyl residues (tRNAmet). Over 85% of the tRNAMet in purified avian myeloblastosis virus consists of one tRNAmt species, which resembles host-cell tRNAMet IV with respect to chromatographic properties on Avian RNA tumor viruses such as avian myeloblastosis virus (AMV), Rous associated virus-1, and Rous sarcoma virus contain a population of functional transfer RNA (tRNA) molecules that differs from the total tRNA of their host cells, both in range and in level of aminoacid-accepting activity (1-5). It has been established that this RNA is neither a contaminant of the preparation nor a degradation product, but is an integral part of the virion (4-6), In addition purified virions of AMV contain aminoacyl-tRNA synthetase activities (7, 8) and a specific tRNA methylase (9), A comparison of the base composition of AMV tRNA with that of both normal and transformed host cells indicates that the viral RNA contains all the minor bases that are present in host tRNA. There is no evidence for additional modifications in the viral RNA, but the distribution of certain minor bases is altered (3). Although there is no direct evidence as to the origin of viral tRNA, it is probable that a selection of a limited population of host-cell molecules takes place to provide the virion tRNA. Alternatively, the tRNAs found in the virus could be unique to the virus, or contain specific modifications of host-cell tRNAs. The experiments described here were done in an attempt to detect unique viral tRNAs, if they exist. Reversed-phase chromatography on RPC-5 absorbent of AMV tRNA charged with single aminoacids reveals that the virus contains particular isoaccepting species of the corresponding host-cell tRNA. In some cases, the ratio of different isoaccepting species is altered (Elder, unpublished). Thus, whereas chick cells contain four peaks of methionyl-tRNA, both AMV and Rous associated virus-i have a single predominating methionyl-tRNA that is chromatographically similar to one of the host tRNAMet species (2). In view of the degree of selectivity and the unique role of methionyl-tRNA in initiation of protein synthesis (10), it was important to determine whether the tRNAMet present in the virus was somehow involved in initiation of viral protein synthesis.Eukaryotic cells contain two classes of methionine-accepting tRNA, tRNAmet and tRNAmet (10). Met-tRNAm, in common with all aminoacyl-tRNAs, donates its amino acid into internal positions of peptides, whereas Met-tRNAf is the initiator tRNA. A distinguishing Teature of the initiator is thatMet-tRNAf donates methionine exclusively into the Nterminal position of newly synthesized proteins. This property results from its specific interaction with initiation factors (11), andj inability to form a stable complex with elongation factor I (12), both of which imply that the structure of tRNAIet ha...

Structure and Properties of an RNA Primer for Initiation of Rous Sarcoma Virus DNA Synthesis In Vitro

“…i:;ii:!~:i:i~ii~i!~:iliiiii:, iiii~i:ii is a minor spot in the cell RNA pattern. This is strong evidence for the nonrandom packaging of cellular RNAs into virions, confirming the conclusions of others (Bonar et al 1967;Trhvnicek 1968Trhvnicek , 1969Carnegie et al 1969;Bishop et al 1970a;Erikson and Erikson 1970;Randerath et al 1971;Wang et al 1973;Elder and Smith 1973;Gallagher and Gallo 1973).…”

“…3) show clear differences. Other laboratories have also observed differences between free and associated small RNAs (Erikson and Erikson 1971;Rosenthal and Zamecnik 1973a,b;Nichols and Waddell 1973;F a r a s et al 1973a;Elder and Smith 1973;Gallagher and Gallo 1973). Figure 2.…”

Cell-free translation of virion RNA from nondefective and transformation-defective Rous sarcoma viruses

“…We have therefore modified the cell-free system in several ways in an attempt to translate proteins other than the gs antigens from the 35S RNA. For example, RSV virions contain a number of tRNA species, both free in the virion and associated with the 70S RNA (8,31). It is possible that the free tRNA's, which are derived from the host cell, are necessary for viral protein synthesis.…”

RNA-directed DNA synthesis by the DNA polymerase of Rous sarcoma virus: structural and functional identification of 4S primer RNA in uninfected cells.

“…In the absence of this RNA primer molecule the template activity of RSV RNA is no better than that of other natural RNA molecules (14,17), suggesting that the preference exhibited by the RNA-directed DNA polymerase for oncornavirus 70S RNA as template is a special feature of the RNA and not a property of the enzyme. Although no virus-specific functions have previously been assigned to any of the other low molecular weight RNA species associated with virions of RNA tumor viruses, many of the virion-associated tRNA species appear to be cellular in origin and selectively incorporated into the virus particle during budding of the virus from the cell membrane (3,(7)(8)(9)(10)(11) (21). These results indicate that the 4S RNA primer molecule required by the RSV RNA-directed DNA polymerase for the initiation of DNA synthesis in vitro is present in normal, uninfected cells, and is in all likelihood a cell-specified molecule.…”

“…The low-molecular-weight population includes 7S, 5S, and 4S RNA and is generally referred to as RSV-free (not associated with 70S RNA) RNA (1)(2)(3)(4)(5)(6). The free 4S RNA exhibits structural features of transfer RNA (tRNA) and can be acylated with amino acids (3,(5)(6)(7)(8)(9)(10)(11). RSV 70S RNA is segmented in nature, containing three to four high-molecular-weight 30-35S RNA subunits (1) and several low-molecular-weight species.…”